Flat revertants of Kirsten sarcoma virus-transformed cells have been isolated which appears to represent mutants in one of the cellular genes involved in transformation by Kirsten sarcoma virus. The revertants were isolated following selection in ouabain, which appears to be much less toxic for these mutants than for their parental transformed cells. Revertants have been fused to a number of transformed cell lines, and the properties of the resulting heterokaryons have been examined in monolayer cultures and in agar suspension. The results show that the revertant phenotype is dominant in heterokaryons involving cells transformed by Kirsten and Harvey sarcoma viruses. On the other hand, fusion of revertant cells with cells transformed by other agents, including other RNA and DNA tumor viruses as well as chemicals results in heterokaryons which are morphologically transformed and grow in soft agar. These results indicate that the revertants described here represent mutants in a cellular gene or genes involved in transformation by Harvey and Kirsten sarcoma viruses. The differential toxic effect of ouabain on Kirsten sarcoma virus-transformed NIH/3T3 cells may indicate the mechanism for transformation by these viruses.